Metal-pipe bonded body, pipe expansion method of metal-pipe bonded body, and method for inspecting metal-pipe bonded body

ABSTRACT

In diffusion bonding a metal pipe  112   e  and a metal pipe  114   e  via a bonded interface  116   e  formed at the end parts, a portion that is inclined with respect to the radial direction of metal pipes  112   e  and  114   e  is provided at least at part of bonded interface  116.  In this case, the inclination angle φ of bonded interface  116   e  and the tip angle 2θ of a pipe expansion tool  130  are preferably in the relationship, 0&lt;φ≦θ+60°. In performing pipe expansion of such a metal pipe bonded body  110   e,  the pipe expansion tool  130  is moved from the metal pipe  112   e,  at which the inclined portion of junction face  116   e  is formed to have a protruding shape, towards the metal pipe  114   e,  at which the inclined portion of junction face  116   e  is formed to have a recessed shape. Furthermore, the inner diameter at the vicinity of the junction face of the metal pipe junction may be made larger than the inner diameter at positions away from the junction face.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a metal-pipe bonded body and a pipe expansion method of metal-pipe bonded body, and to more particularly relates to an expandable metal-pipe bonded body and a pipe expansion method of metal-pipe bonded body that are favorable for plant piping and line pipes used in the chemical industry, petrochemical industry, etc., and for casing tubes, production tubes, coiled tubes, and other oil well pipes used in oil wells.

[0002] In addition to this, the present invention also relates to a metal pipe bonded body inspection method, and more particularly relates to a metal pipe bonded body inspection method that is favorable as a nondestructive inspection method for a metal pipe bonded body having a inclined part at the bonded interface and is liquid phase diffusion bonded.

[0003] Since in the fields of chemical industry, petrochemical industry, etc., long-size metal pipes have been conventionally used for obtaining the targeted products by use of chemical reactions under various environments and for transporting the chemical reaction raw materials, intermediate products, targeted products, and other corrosive fluids over a long distance.

[0004] Though seamless steel pipes, which are excellent in corrosion resistance, are generally used as metal pipes that are exposed to corrosive environments, industrially mass-produced seamless steel pipes are 10 to 15 m in length and the upper limit for production is approximately 100 m. Thus in such cases, bonded bodies (referred to hereinafter as “metal pipe bonded bodies”), with which a plurality of seamless steel pipes of 10 to 15 m length are joined, are used.

[0005] Conventional methods for joining metal pipes include the screw connecting method (mechanical coupling method), welding methods (orbital welding method), friction welding method, diffusion bonding method, etc. Among these, the diffusion bonding method provides the advantages that a joint, which is high in air-tightness and is of a bonding strength that is equivalent to the base material strength, can be obtained and the working time is short in comparison to the welding method. The diffusion bonding method is therefore anticipated for application as a method for joining oil well pipes, line pipes, etc.

[0006] Also, though a metal pipe bonded body is generally used as it is in the joined condition, depending on the application, the process of expanding the inner diameter of the metal pipe bonded body (this process shall be referred to hereinafter as “pipe expansion”) is carried out after the joining process in some cases. For example, recently in the field of oil well pipes, a method of performing pipe expansion after the burying of a metal pipe bonded body underground has been proposed for reducing the drilling cost of oil wells.

[0007] For example, a method of burying a casing, made of malleable material, in a bore hole drilled into the ground and expanding a hydraulic expanding tool inside the interior of the casing to expand the casing in the radial direction with respect to the bore hole wall has been disclosed in Japanese Patent Publication No. 507610 of 1995.

[0008] Also, International Patent Publication No. WO 98/0062, based on the Patent Cooperation Treaty, discloses a method of inserting a steel pipe, which is made of a malleable type of steel that undergoes strain hardening without undergoing necking or ductile breaking, inside a gallery or a previously buried casing, and pipe expanding the casing using a mandrel made of nonmetallic material and having a tapered surface.

[0009] In applications of pipe expansion of a metal pipe bonded body to oil well pipes, the rate of expansion of the inner diameter before and after pipe expansion (shall be referred to hereinafter as the “pipe expansion rate”) must be at least 5% and is more preferably 20% or more for reducing the drilling costs of oil wells. On the other hand, the diffusion bonding method is, as has been mentioned above, a joining method that enables the production of high-quality metal pipe bonded bodies at high efficiency. Significant reductions in oil well drilling costs can thus be anticipated by combining the diffusion bonding method with pipe expansion and applying this to oil well pipes.

[0010] However, generally in the diffusion bonding of metal pipes, butt joining is performed upon forming the bonded interface by processing the end faces of the metal pipes to be perpendicular to the axial direction. Thus when pipe expansion is performed on such a metal pipe bonded body as it is, the shearing force that acts on the bonded interface during pipe expansion increases in accompaniment with the increase of the pipe expansion rate and this can lead to the formation of cracks at the bonded interface.

[0011] Conventionally, as a bonding method for a plurality of metal pipes that constitute a metal pipe bonded body, a liquid phase diffusion bonding method is known. Due to being lower in heat deformation than the welding method and enabling a joint of the same quality as the base material to be obtained in a short time, the liquid phase diffusion bonding method is used as the method for joining such metal pipes as plant piping, line pipes, oil well pipes, etc. However, the quality of a bonded body made by the liquid phase diffusion bonding method depends sensitively on the unavoidable variations of the bonding conditions, the expertise of the worker, etc. and flaws can occur at the bonded interface. Thus when metal pipes are joined by the liquid phase diffusion bonding method, the existence of flaws at the bonded interface must be inspected in a nondestructive manner in order to assure the quality of the metal pipe bonded body that is obtained.

[0012] The X-ray transmission test method or the ultrasonic flaw detection test method, etc. is generally used as a nondestructive inspection method for a bonded body. Among these, the X-ray transmission test method is good for the inspection of three-dimensional flaws, such as blow holes, etc. Meanwhile, the ultrasonic flaw detection test method is good for the inspection of planar flaws, such as cracks, etc. In the case of the liquid phase diffusion bonding method, since the flaws that occur at the bonded interface are normally planar flaws, such as cracks, bonding faults, etc., the ultrasonic flaw detection method is used for inspection of a metal pipe bonded body made by liquid phase diffusion bonding.

[0013] Also, generally when metal pipes are liquid phase diffusion bonded, the end parts of the metal pipes are processed to be perpendicular to the axial direction and butt joining is performed. Thus in the detection of the existence of a flaw at the bonded interface, the oblique flaw detection method is generally employed in which the ultrasonic wave is made incident obliquely on the bonded interface and the existence of a flaw is judged from the magnitude of the reflected wave.

[0014] However, the conventional oblique flaw detection method is limited in terms of high-precision quantitative inspection of the magnitudes, positions, and shapes of the flaws that occur at the bonded interface.

[0015] Also, the quality of a metal pipe bonded body is not only affected by the poor fusing that occurs at the bonded interface but is also affected by the offset that is formed at the end part of the bonded interface, the crystalline structure in the vicinity of the joined parts, etc. Also, molten insert material may leak out from the bonded interface and since the resulting solidified parts are brittle, they tend to be starting points for the concentration of stress. Therefore such solidified parts must also be inspected in a nondestructive manner in order to assure the quality of the metal pipe bonded body.

SUMMARY OF THE INVENTION

[0016] It is an object that this invention attempts to achieve is to provide a metal-pipe bonded body, with which the possibility of crack formation at the bonded interface is low even when pipe expansion of a high pipe expansion rate is carried out. It is another object of the present invention to provide a pipe expansion method of metal-pipe bonded body, which enables pipe expansion of a high pipe expansion rate to be performed without causing cracks to form at the bonded interface.

[0017] The object can be achieved by a metal-pipe bonded body, according to the present invention, having a plurality of metal pipes which are diffusion bonded via a bonded interface formed at the end parts thereof. In the metal-pipe bonded body, at least part of the bonded interface is inclined with respect to the radial direction of the metal pipes.

[0018] Since with the metal-pipe bonded body of this invention, at least part of the diffusion bonded bonded interface is inclined with respect to the radial direction of the metal pipes, the shearing force that is generated at the bonded interface in the process of pipe expansion can be alleviated by moving a pipe expanding tool from one metal pipe, with which the inclined part of the bonded interface is formed to have a protruding shape, towards another metal pipe, with which the inclined part of the bonded interface is formed to have a recessed shape. Pipe expansion of the metal pipe bonded body can thus be performed without the formation of cracks at the bonded interface.

[0019] It is a further object of the present invention to provide a metal pipe bonded body inspection method by which flaws, which have occurred at the bonded interface of a metal pipe bonded body, can be detected at high precision. It is another object of the present invention to provide a metal pipe bonded body inspection method by which the magnitude of the offset that has formed at the end part of the bonded interface, the appropriateness of the crystalline structure in the vicinity of the bonded interface, and the existence of insert material that has leaked out and solidified at the end part of the bonded interface can be inspected.

[0020] In order to achieve the above objects, this invention provides in a metal pipe bonded body inspection method for inspecting, by the ultrasonic flaw detection method, a metal pipe bonded body at which a plurality of metal pipes are joined by liquid phase diffusion bonding, a metal pipe bonded body inspection method being characterized in that the bonded interface of the metal pipe bonded body has a inclined part and in having a first flaw detection process, in which an ultrasonic wave is made incident perpendicularly on the inclined part and the reflection echo that is reflected from the inclined part is detected.

[0021] Since the method of making an ultrasonic wave perpendicularly incident on the inclined part and detecting the reflection echo is employed in the metal pipe bonded body inspection method of this invention, the maximum sensitivity can be obtained. Also, by focusing the ultrasonic wave and scanning in the circumferential direction or longitudinal direction, the flaw information on the inclined part can be obtained as 2-dimensional information, and the magnitudes, positions, shapes, etc. of flaws can be obtained from this flaw information.

[0022] Also in the case where the bonded interface has a perpendicular part, the inspection method preferably has furthermore a second flaw detection process, in which an ultrasonic wave is made obliquely incident on the perpendicular part and the reflection echo that is reflected from the perpendicular part is detected. The existence of flaws at the perpendicular parts of the bonded interface may thereby be inspected.

[0023] The inspection method may also have furthermore a first pipe thickness measurement process, in which an ultrasonic wave is made perpendicularly incident on one of the metal pipes that are disposed adjacent each other across the bonded interface and the pipe thickness of the one of the metal pipes is measured from the difference in the arrival times of the surface echo and the bottom face echo, a second pipe thickness measurement process, in which an ultrasonic wave is made perpendicularly incident on the other of the metal pipes that are disposed adjacent each other across the bonded interface and the pipe thickness of the other of the metal pipes is measured from the difference in the arrival times of the surface echo and the bottom face echo, a surface offset measurement process, in which the surface offset between the one of the metal pipes and the other of the metal pipes is measured, and an inner face offset calculation process, in which the inner face offset between the one of the metal pipes and the other of the metal pipes is determined from the pipe thickness and the surface offset of the one of the metal pipes and the other of the metal pipes. The inner face offset of the metal pipe bonded body can thereby be measured in a non-destructive manner.

[0024] Also, the inspection method may furthermore have a backward scattering intensity measurement process, in which an ultrasonic wave is transmitted in the direction of the interior of the metal pipe at the vicinity of the bonded interface and the backward scattering intensity is measured, and a crystal grain diameter calculation process, in which a previously determined correlation between the crystal grain diameter of the metal pipe and the backward scattering intensity is used to determine the size of the crystal grain from the magnitude of the backward scattering intensity that was measured in the backward scattering intensity measurement process. The crystalline structure in the vicinity of the bonded body can thereby be judged.

[0025] The inspection method may furthermore have a solidified phase detection process, in which an ultrasonic wave is made obliquely incident towards the inner face end part of the metal pipe bonded body and the position of the reflection echo that is reflected from the inner face of the metal pipe bonded body is measured. The existence of a solidified phase at the inner face end part can thereby be inspected in a non-destructive manner.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1(a) is a sectional view of the vicinity of the bonded interface of a prior-art metal pipe bonded body and FIGS. 1(b) to 1(f) are sectional views of the vicinities of the bonded interfaces of metal pipe bonded bodies of the first embodiment of this invention;

[0027]FIG. 2 is a process diagram, which illustrates an example of the method of producing a metal pipe bonded body of the first embodiment of this invention;

[0028]FIG. 3 is a diagram for explaining the relationship between the inclination angle φ and the tip angle 2θ;

[0029]FIG. 4 is a process diagram, which illustrates an example of the method of producing a metal pipe bonded body of the second embodiment of this invention;

[0030]FIG. 5 is a diagram for explaining the method for inspecting for flaws that have occurred at the inclined part;

[0031]FIG. 6 is a diagram, which shows the results of inspection of the inclined part of a metal pipe bonded body using the method shown in FIG. 5;

[0032]FIG. 7 is a diagram for explaining the method for inspecting for flaws that have occurred at the perpendicular part;

[0033]FIG. 8 is a diagram for explaining the method for measuring the inner face offset of the bonded interface;

[0034]FIG. 9 is a diagram for explaining the method for judging the crystalline structure in the vicinity of the bonded interface; and

[0035]FIG. 10 is a diagram for explaining the method for detecting a solidified phase that has formed at the inner face end part of the bonded interface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Embodiments of this invention shall now be described in detail. The expandable metal-pipe bonded body of the first embodiment of this invention is one with which a plurality of metal pipes are diffusion bonded via a bonded interface formed at the end parts thereof and is characterized in that at least part of the bonded body bonded interface is inclined with respect to the radial direction of the metal pipes.

[0037]FIG. 1(a) is a sectional view of a prior-art metal pipe bonded body. Each of FIGS. 1(b) to 1(f) shows an example of a sectional view of a metal pipe bonded body of the present embodiment. As shown in FIG. 1(a), with the prior-art metal pipe bonded body 10 a, metal pipes 12 a and 14 a are joined at their end faces and the bonded interface 16 a is perpendicular to the axial direction of metal pipes 12 a and 14 a.

[0038] Meanwhile, with the metal pipe bonded body 10 b shown in FIG. 1(b), metal pipes 12 b and 14 b are joined at their end faces, and bonded interface 16 b is comprised of a tapered part 18 b, which is inclined with respect to the radial direction of metal pipes 12 b and 14 b, and a first flat part 20 b, which is formed at the outer peripheral sides of metal pipes 12 b and 14 b.

[0039] Also, with the metal pipe bonded body 10 c shown in FIG. 1(c), metal pipes 12 c and 14 c are joined at their end faces, and bonded interface 16 c is comprised of a tapered part 18 c, which is inclined with respect to the radial direction of metal pipes 12 c and 14 c, and a second flat part 22 c, which is formed at the inner peripheral sides of metal pipes 12 c and 14 c.

[0040] With the metal pipe bonded body 10 d shown in FIG. 1(d), metal pipes 12 d and 14 d are joined at their end faces, and bonded interface 16 d is comprised of a tapered part 18 d, which is inclined with respect to the radial direction of metal pipes 12 d and 14 d, and a first flat part 20 d and a second flat part 22 d, which are formed at the outer peripheral sides and inner peripheral sides, respectively, of metal pipes 12 d and 14 d.

[0041] With the metal pipe bonded body 10 e shown in FIG. 1(e), metal pipes 12 e and 14 e are joined at their end faces, and bonded interface 16 e is comprised only of a tapered part, which is inclined with respect to the radial direction of metal pipes 12 e and 14 e.

[0042] Furthermore, with the metal pipe bonded body 10 f shown in FIG. 1(f), metal pipes 12 f and 14 f are joined at their end faces, and bonded interface 16 f is comprised of a curved surface part, with which the angle of the portion that is inclined with respect to the radial direction of metal pipes 12 f and 14 f varies in a continuous manner.

[0043] The angle (φ) of the tapered part with respect to the radial direction of the metal pipes (referred to hereinafter as the “inclination angle”) must be at least greater than 0. Though the formation of cracks at the bonded interface will be less likely the greater this inclination angle φ, processing will be made difficult if the inclination angle φ is too great. Thus if the tip angle of a pipe expansion tool is 20θ, the inclination angle φ is preferably in the range expressed by the following Formula 1.

0<φ≦θ+60 °  [Formula 1]

[0044] In the case where the portion of the bonded interface that is inclined with respect to the radial direction of the metal pipes is a curved surface part as shown in FIG. 1(f), the “inclination angle (φ)” refers to the value approximated by the angle formed by the plane formed by joining both ends of the curved surface part with a straight line and the radial direction of the metal pipes.

[0045] Diffusion bonding methods include the solid phase diffusion bonding method, with which the elements are made to undergo diffusion while maintaining the solid phase condition, and the liquid phase diffusion bonding method, with which an insert material is inserted in the bonded body interface and the insert material is melted and a part of the components thereof are made to undergo diffusion into the metal pipe side. The metal pipe bonded body by this invention may be made by either of these methods.

[0046] The method of producing the metal pipe bonded body of the present embodiment shall now be described. FIG. 2 is a process diagram, which illustrates an example of the method of producing the metal pipe bonded body 10 e shown in FIG. 1(e). As shown in FIG. 2(a), a protruding type tapered part and a recessed type tapered part are formed at the end faces of metal pipe 12 e and metal pipe 14 e, respectively, and these are butt joined via an insert material 40.

[0047] Next, as shown in FIG. 2(b), metal pipes 12 e and 14 e are fixed by clamps 42 and 44, respectively, and a prescribed pressure is applied to an interface to be bonded. Also, a coil 46 is positioned around the interface and the surroundings of coil 46 are sealed closely by a shield 48. Next, a high frequency current is applied to coil 46 while keeping the interior of shield 48 in a non-oxidizing atmosphere. The vicinity of the interface is then heated to the bonding temperature and after maintaining this temperature for a prescribed amount of time, cooling is performed to obtain the metal pipe bonded body 10 e, with which the bonded interface 16 e is comprised only of a tapered part.

[0048] A metal pipe bonded body having a flat part at part of the bonded interface or a metal pipe bonded body, with which a part of the bonded interface is a curved surface part of a prescribed radius of curvature, may also be produced by the same procedure. In the case where bonding by the solid phase diffusion bonding method is to be performed, the metal pipes are butt joined directly and heated without the insertion of insert material 40.

[0049] The actions of the metal pipe bonded body of the present embodiment shall now be described. By inserting a pipe expansion tool, with a tip angle of 2θ, into the interior of the metal pipe bonded body and moving the pipe expansion tool from one side of the metal pipe bonded body towards the other side of the metal pipe bonded body, the inner diameter of the metal pipe bonded body can be expanded uniformly by the pipe expansion tool.

[0050] However, in the process of moving the pipe expansion tool, a part of the force that acts on the pipe expansion tool acts in the radial direction of the metal pipe bonded body. Thus if the bonded interface is parallel to the radial direction of the metal pipe (that is, if the inclination angle φ=0), the force that acts in the radial direction will act as it is as a shearing force on the bonded interface, and especially in the case where the pipe expansion rate is large, cracks may form in the bonded interface.

[0051] In contrast, when for example as shown in FIG. 3, an expansion tool 30, with a tip angle of 2θ, is inserted into the interior of metal pipe bonded body 10 e, with which the bonded interface 16 e is comprised only of a tapered part, and the pipe expansion tool 30 is moved from the metal pipe 12 e, with which the end face has a protruded shape, towards the metal pipe 14 e, with which the end face has a recessed shape, only a part of the force that acts in the radial direction acts on bonded interface 16 e. The shearing force that acts on bonded interface 16 e in the process of pipe expansion is therefore alleviated and the formation of cracks at the bonded interface is restrained. The same applies in the case where part of the bonded interface is a flat part and in the case where part of the bonded interface is a curved surface part of a prescribed radius of curvature.

[0052] The metal pipe bonded body of a second embodiment of this invention shall now be described. The metal pipe bonded body of this embodiment is characterized in that at least part of the bonded interface is inclined with respect to the radial direction of the metal pipes and the inner diameter in the vicinity of the bonded interface is larger than the inner diameter at positions away from the bonded interface.

[0053] Here, the increase of the inner diameter in the vicinity of the bonded interface with respect to the inner diameter at positions away from the bonded interface (referred to hereinafter as the “diameter expansion rate”) should be determined in accordance with the material of the metal pipes, the deformability of the vicinity of the bonded interface after joining, the tube expansion rate achieved by the tube expansion tool, etc. Generally, the closer the diameter expansion rate is to the pipe expansion rate, the more preferable since the formation of cracks during pipe expansion can then be restrained.

[0054]FIG. 4 is a process diagram, which illustrates an example of a method of producing the metal pipe bonded body of the present embodiment. As shown in FIG. 4(a), metal pipes 12 g and 14 g of uniform inner diameter are prepared. Then using an appropriate tool, the diameter expansion of the end parts of metal pipes 12 g and 14 g by a prescribed diameter expansion rate is performed using an appropriate tool as shown in FIG. 4(b).

[0055] Then as shown in FIG. 4(c), tapered parts are formed at the end faces of the diameter expanded metal pipes 12 g and 14 g. FIG. 4(c) shows the case where a surface (to be bonded) 16 g with a protruding type tapered part is formed at the end face of metal pipe 12 g and a surface (to be bonded) 16 g′ with a recessed type tapered part is formed at the end face of metal pipe 14 g. The metal pipes 12 g and 14 g thus obtained are butt joined directly or upon inserting an insert material and diffusion bonded in the same manner as the first embodiment described above to obtain a metal pipe bonded body with which the inner diameter in the vicinity of a bonded interface defined by mated surfaces 16 g, 16 g′ is expanded.

[0056] With the metal pipe bonded body of this embodiment, since at least part of the bonded interface is inclined with respect to the radial direction of the metal pipe and since the inner diameter in the vicinity of the bonded interface is made greater than the inner diameter at positions away from the bonded interface, the force that acts in the radial direction when a pipe expansion tool passes near the bonded interface is made even smaller than in the case where diameter expansion of the end parts is not performed. The shearing force that acts on the bonded interface during pipe expansion is therefore alleviated and the formation of cracks at the bonded interface is restrained further.

[0057] [Examples]

[0058] (Example 1)

[0059] Liquid phase diffusion bonding was performed using metal pipes of different bonded interface shapes. L80 carbon steel pipes (API 5CT) of 177.8 mm outer diameter, 9.19 mm stock thickness, and 5 m length were used as the metal pipes. The shapes shown in FIGS. 1(a) to 1(e) were used as the shapes of the bonded interfaces, and liquid phase diffusion bonding was performed without performing diameter expansion of the end parts.

[0060] The bonding conditions are as follows: Insert material: BNi-3 (t40 μm, JISZ3265) Bonding temperature: 1250° C. Holding time:  180s Compression force:   3 MPa Bonding atmosphere: Ar

[0061] Five (5) each of metal pipe bonded bodies of different bonded interface shapes were made under the bonding conditions given above and pipe expansion was performed on each of these metal pipe bonded bodies. For pipe expansion, a pipe expansion tool, with a tip angle 2θ of 30°, was inserted into the interior of the metal pipe bonded body and was moved using hydraulic pressure at a travel speed of 10 m/min. In all cases of the present example, the pipe expansion tool was moved from a metal pipe, with which the end face was formed to have a protruding shape, towards a metal pipe, with which the end face was formed to have a recessed shape (hereinafter, this direction of movement shall be referred to as the “up→down” direction and the opposite direction of movement shall be referred to as the “down→up” direction). Also in all cases of the present examples the pipe expansion rate was set to 20%. Dye penetrant tests were then performed on the inner and outer surfaces of the bonded body parts of the metal pipe bonded bodies on which pipe expansion was performed. Furthermore, tension tests, using a universal tester (500Tonf), were performed to evaluate the stability of the tensile characteristics The results are shown in Table 1. TABLE 1 Experiment No. 1 2 3 4 5 Joint Shape Fig. Fig. Fig. Fig. Fig. 1 (a) 1 (b) 1 (c) 1 (d) 1 (e) φ (°) 0 30 30 30 30 Rmax (μm) 12 12 12 12 12 Pipe expansion conditions 2θ (°) 30 30 30 30 30 Pipe 20 20 20 20 20 expansion rate (%) Tool travel 10 10 10 10 10 speed (m/min) Tool Up → Up → Up → Up → Up → movement Down Down Down Down Down direction Characteristic of the Pipe-expanded metal pipe bounded body Dye penetrant test results 1 Outer Good Good Good Good Good surface Inner Good Good Good Good Good surface 2 Outer Good Good Good Good Good surface Inner Good Good Good Good Good surface 3 Outer Good Good Good Good Good surface Inner Good Good Good Good Good surface 4 Outer Good Good Good Good Good surface Inner Good Good Good Good Good surface 5 Outer Good Good Good Good Good surface Inner Microsc Good Good Good Good surface opic cracks Tension test results Tensile strength (MPa) 1 786 785 783 791 792 2 790 791 789 788 788 3 788 784 784 785 786 4 784 787 785 789 787 5 742 790 787 790 785 Fractured Portion 1 Base Base Base Base Base material material material material material 2 Base Base Base Base Base material material material material material 3 Base Base Base Base Base material material material material material 4 Base Base Base Base Base material material material material material 5 Junction Base Base Base Base face material material material material Evaluation Δ ∘ ∘ ∘ ∘

[0062] In the case of Experiment No. 1, with which the bonded interface was processed to be perpendicular with respect to the axial direction of the metal pipes as shown in FIG. 1(a), cracks in the vicinity of the bonded interface were not observed or identified in four of the five samples prepared in the dye penetrant test. These samples also exhibited tensile strengths of 780 to 790MPa and all test pieces were fractured in the base material. However, microscopic cracks were found at the inner face side of the bonded body part of one of the five samples. This sample also was fractured at the bonded interface in the tensile test and the tensile strength was decreased to approximately 740MPa.

[0063] On the other hand, in the cases of Experiment No. 2 to Experiment No. 5, with which a tapered part (inclination angle φ=30°) was provided at least at part of the bonded interface as shown in FIGS. 1(b) to 1(e), cracks in the vicinity of the bonded interface were not observed in any of the five samples prepared. Also, tensile strength values of 780 to 790MPa were obtained in a stable manner in all cases and all test pieces were fractured in the base material.

[0064] (Example 2)

[0065] Liquid phase diffusion bonding was performed using metal pipes that differed in bonded interface shapes. The same metal pipes as those of Example 1 were used and diameter expansion of the end parts by a diameter expansion rate of 10 to 20% was performed on each of these metal pipes. Then after processing the bonded interfaces to the shapes shown in FIGS. 1(b) to 1(f), liquid phase diffusion bonding was performed The bonding conditions were the same as those of Example 1.

[0066] Five (5) each of metal pipe bonded bodies of different bonded interface shapes were made under the bonding conditions and pipe expansion were performed on each of these metal pipe bonded bodies. For pipe expansion, a pipe expansion tool, with a tip angle 2θ of 30°, was inserted into the interior of the metal pipe bonded body and was moved using hydraulic pressure at a travel speed of 10 m/min. With the present example, the pipe expansion tool was moved in the “up→down” direction. Also in all cases of the present example, the pipe expansion rate was set to 25%. Dye penetrant tests were then performed on the inner and outer surfaces of the bonded body parts of the metal pipe bonded bodies on which pipe expansion was performed. Furthermore, tension tests, using a universal tester (500Tonf) were performed to evaluate the stability of the tensile characteristics. The results are shown in Table 2. TABLE 2 Experiment No. 6 7 8 9 10 Diameter 10% 10% 15% 15% 20% expansion rate of metal pipe end parts Joint Shape Fig. Fig. Fig. Fig. Fig. 1 (b) 1 (c) 1 (d) 1 (e) 1 (f) φ (°)  30  30  30  30  30* Rmax (μm)  12  12  12  12  12 Pipe expansion conditions 2θ (°)  30  30  30  30  30 Pipe  25  25  25  25  25 expansion rate (%) Tool travel  10  10  10  10  10 speed (m/min) Tool Up → Up → Up → Up → Up → movement Down Down Down Down Down direction Characteristic of the Pipe-expanded metal pipe bonded body Dye penetrant test results 1 Outer Good Good Good Good Good surface Inner Good Good Good Good Good surface 2 Outer Good Good Good Good Good surface Inner Good Good Good Good Good surface 3 Outer Good Good Good Good Good surface Inner Good Good Good Good Good surface 4 Outer Good Good Good Good Good surface Inner Good Good Good Good Good surface 5 Outer Good Good Good Good Good surface Inner Good Good Good Good Good surface Tension test recults Tensile strength (MPa) 1 813 811 816 814 811 2 811 815 813 819 813 3 809 820 811 810 816 4 814 816 817 816 817 5 810 814 815 813 814 Fractured Portion 1 Base Base Base Base Base material material material material material 2 Base Base Base Base Base material material material material material 3 Base Base Base Base Base material material material material material 4 Base Base Base Base Base material material material material material 5 Base Base Base Base Base material material material material material Evaluation ∘ ∘ ∘ ∘ ∘

[0067] With the present example, since at least part of the bonded interface was provided with a tapered part (inclination angle φ=30°) as shown in FIG. 1(b) to 1(f), cracks were not observed in the vicinity of the bonded interface with all five samples. Also, tensile strength values of 810 to 820MPa were obtained in a stable manner in all cases and all test pieces were fractured in the base material.

[0068] (Example 3)

[0069] Liquid phase diffusion bonding was performed using metal pipes of different bonded interface shapes. ASTM A106 carbon steel pipes of 139.7 mm outer diameter, 6.99 mm stock thickness, and 5 m length were used as the metal pipes. The shapes shown in FIGS. 1(b), 1(d), and 1(e) were used as the shapes of the bonded interfaces, and for the sample having the bonded interface shape of FIG. 1(b), liquid phase diffusion bonding was performed without performing diameter expansion of the end parts. With the samples having the bonded interface shape of FIG. 1(d) or FIG. 1(e), liquid phase diffusion bonding was performed upon diameter expansion of the end parts by a diameter expansion rate of 10% and subsequent finishing of the bonded interfaces to the prescribed shapes.

[0070] The bonding conditions are as follows: Insert material: BNi-5 (t50 μm, JISZ3265) Bonding temperature: 1300° C. Holding time:  180s Compression force:   3 MPa Bonding atmosphere: N₂

[0071] Five (5) each of metal pipe bonded bodies of different bonded interface shapes were made under the bonding conditions given above and pipe expansion were performed on each of these metal pipe bonded bodies. For pipe expansion, a pipe expansion tool, with a tip angle 2θ0 of 30°, was inserted into the interior of the metal pipe bonded body and was moved using hydraulic pressure at a travel speed of 10 m/min. With the present example, the pipe expansion tool was moved in the “down→up” direction only in the case of Experiment No. 15, and the pipe expansion tool was moved in the “up→down” direction for the rest of the samples. Also in all cases of the present example, the pipe expansion rate was set to 15%. Dye penetrant tests were then performed on the inner and outer surfaces of the bonded body parts of the metal pipe bonded bodies on which pipe expansion was performed. Furthermore, tension tests, using a universal tester (500Tonf), were performed to evaluate the stability of the tensile characteristics. The results are shown in Table 3. TABLE 3 Experiment No. 11 12 13 14 15 Diameter Not Not 10% 10% 10% expansion rate of expand- expand- metal pipe end ed ed parts Joint Shape Fig. Fig. Fig. Fig. Fig. 1 (b) 1 (b) 1 (d) 1 (e) 1 (e) φ (°)  80  60  45  30  30 Rmax (μm)  12  12  12  12  12 Pipe expansion conditions 2θ (°)  30  30  30  30  30 Pipe  15  15  15  15  15 expansion rate (%) Tool travel  10  10  10  10  10 speed (m/min) Tool Up → Up → Up → Up → Down → movement Down Down Down Down Up direction Characteristic of the Pipe-expanded metal pipe bonded body Dye penetrant test results 1 Outer Good Good Good Good Good surface Inner Microsc Good Good Good Good surface opic cracks 2 Outer Good Good Good Good Good surface Inner Good Good Good Good Microsc surface opic cracks 3 Outer Good Good Good Good Good surface Inner Good Good Good Good Good surface 4 Outer Good Good Good Good Good surface Inner Good Good Good Good Good surface 5 Outer Good Good Good Good Good surface Inner Microsc Good Good Good Good surface opic cracks Tension test results Tensile strength (MPa) 1 468 477 475 478 471 2 472 474 472 477 473 3 478 478 470 476 475 4 473 475 471 480 470 5 459 476 477 479 474 Fractured Portion 1 Junction Base Base Base Base face material material material material 2 Base Base Base Base Junction material material material material face 3 Base Base Base Base Base material material material material material 4 Base Base Base Base Base material material material material material 5 Junction Base Base Base Base face material material material material Evaluation Δ ∘ ∘ ∘ Δ

[0072] With three of the five samples prepared in Experiment No. 11, in which the bonded interface was made to have the shape shown in FIG. 1(b) and have an inclination angle of φ=80°, cracks were not found in the vicinity of the bonded interface by the dye penetrant test. These samples also exhibited a tensile strength of approximately 470MPa and all test pieces were fractured in the base material. However, microscopic cracks were found at the inner face side of the bonded body part of two of the five samples. These samples also were fractured at the bonded interface and were found to be somewhat lowered in tensile strength in the tension test.

[0073] On the other hand, with all five samples prepared in Experiment No. 12, in which the bonded interface was made to have the shape shown in FIG. 1(b) and have an inclination angle of φ=60°, cracks were not found in the vicinity of the bonded interface. Also with all of these samples, tensile strength values of 470 to 480MPa were obtained in a stable manner and all test pieces were fractured in the base material.

[0074] With all five samples prepared respectively in Experiment No. 13 and Experiment No. 14, in which the bonded interface was made to have the shape shown in FIG. 1(d) or FIG. 1(e) upon performing diameter expansion of the metal pipe end parts by a diameter expansion rate of 10%, cracks were not found in the vicinity of the bonded interface. Also with all of these samples, tensile strength values of 470 to 480MPa were obtained in a stable manner and all test pieces were fractured in the base material.

[0075] Meanwhile, with four of the five samples prepared in Experiment No. 15, in which the tool was moved in the “down→up” direction, cracks were not found in the vicinity of the

[0076] bonded interface by the dye penetrant test. These samples also exhibited a tensile strength of approximately 470MPa and all test pieces were fractured in the base material. However, microscopic cracks were found at the inner face side of the junction part of one of the five samples. This sample was also fractured the bonded interface in the tension test.

[0077] Though embodiments of this invention were described in detail above, this invention is by no means limited to the above embodiments and various modifications are possible within scope that does not fall outside the gist of this invention.

[0078] For example, though examples to which this invention was applied to carbon steel pipes were described in the above description of the embodiments, the metal pipes used in this invention are not limited to carbon steel pipes and this invention can be applied to metal pipes of various materials.

[0079] Also, though this invention is especially favorable for oil well pipes, the application of this invention is not limited to oil well pipes and may also be applied to line pipes and various forms of plant piping.

[0080] Hereinafter, the metal pipe junction inspection method according to the present invention shall now be described. This invention concerns a method of inspecting, by use of the ultrasonic flaw detection method, the qualities of a metal pipe bonded body made by liquid phase diffusion bonding. The qualities to be evaluated specifically include the existence of flaws at the bonded interface of a metal pipe bonded body having a inclined part and a perpendicular part, the size of the offset that has formed at the inner face end part of the bonded interface, the diameters of crystal grains in the vicinity of the bonded interface, and the existence of a solidified phase at the inner face end part of the bonded interface.

[0081] First, the process of inspecting for flaws at the inclined part of the bonded interface of a metal pipe bonded body having a inclined part and a perpendicular part (first flaw detection process) shall be described. FIG. 5 is a schematic view, which illustrates the method of detecting a flaw that has occurred at the inclined part. In FIG. 5, metal pipe bonded body 110 is made by the liquid phase diffusion bonding of a plurality of metal pipes 112 _(n) (n=1, 2 - - - ) and of these pipes, two metal pipes 112 _(j) and 112 _(i+1) are shown as examples.

[0082] As shown in FIG. 5, the right end of metal pipe 112 _(i) has been processed to have a recessed form and the left end of metal pipe 112 _(i+1) has been processed to have a protruding form. Also, the bonded interface has a inclined part 114 ₁ and perpendicular parts 116 _(i) and 118 _(i). The metal pipes 112 _(i) and 112 _(i+1) are joined by butt joining the right end of the metal pipe 112 _(i) that has been processed to have a recessed form with the left end of the metal pipe 112 _(i+1) that has been processed to have a protruding form upon interposing an insert material (not shown) in between and maintaining a temperature that is equal to or greater than the melting point of the insert material and yet less than the melting point of the metal pipes 112 _(i) and 112 _(i+1).

[0083] To detect a flaw that has occurred at inclined part 114 _(i), first an ultrasonic probe 120 is positioned at a prescribed distance from metal pipe bonded body 110. Here, a focusing-type probe is preferably used as the ultrasonic probe 120. The use of a focusing-type probe enables minute flaws to be identified and provides the advantages of improved sensitivity and S/N ratio. A suitable contact medium is interposed between metal pipe bonded body 110 and ultrasonic probe 120. Normally, water is used as the contact medium. Next, an ultrasonic wave is made perpendicularly incident on inclined part 114 _(i) from ultrasonic probe 120. The angle of incidence of the ultrasonic wave on metal pipe bonded body 110 should be set in consideration of the sound velocity difference between the contact medium and the metal pipes 112 ₁ and 112 _(i+1).

[0084] Here, if inclined part 114 _(i) does not have a flaw, since the incident ultrasonic wave will pass right through inclined part 114 _(i) and be reflected by the inner face of metal pipe bonded body 110, a reflection echo will not be detected by ultrasonic probe 120. On the other hand, if there is a flaw at inclined part 114 _(i), since the reflection echo that has been reflected by the flaw will travel along substantially the same path as the incident wave and be received by ultrasonic probe 120, it can readily be known that there is a flaw at inclined part 114 _(i).

[0085] Here, the existence of flaws at inclined part 114 _(i) can be inspected across the entire circumference of metal pipe bonded body 110 by rotating metal pipe bonded body 110 in one direction. Furthermore, by scanning the ultrasonic probe 120 in the longitudinal direction of metal pipe bonded body 110 at the same time as rotating metal pipe bonded body 110 in one direction, the reflection echo can be detected as two-dimensional information, thereby enabling not only the existence of flaws to be inspected across the entire surface of inclined part 114 _(i) but the sizes, positions, and shapes of the flaws to be evaluated quantitatively. An example of a result of inspecting for the existence of flaws in the circumferential direction is shown in FIG. 6. From FIG. 6, it can be understood that a flaw echo is detected at the position of approximately 270 degrees.

[0086] Next, the process of inspecting for the existence of flaws at the perpendicular part of the bonded interface of the metal pipe bonded body having a inclined part and a perpendicular part (second flaw detection process) shall be described. FIG. 7 is a schematic view, which shows the method of detecting a flaw that has occurred at the perpendicular part. To detect the existence of a flaw at perpendicular part 116 _(i), first the ultrasonic probe 120 is positioned at a prescribed distance from metal pipe bonded body 110 as shown in FIG. 7 and a contact medium is interposed between metal pipe bonded body 110 and ultrasonic probe 120. Next, an ultrasonic wave is made incident obliquely towards perpendicular part 116 _(i) from ultrasonic probe 120.

[0087] If perpendicular part 116 _(i) does not have a flaw, since the incident ultrasonic wave will pass right through perpendicular part 116 _(i) and be reflected by the inner face of metal pipe bonded body 110, a reflection echo will not be detected by ultrasonic probe 120. On the other hand, if there is a flaw at perpendicular part 116 _(i), since the reflection echo that has been reflected by the flaw will travel along substantially the same path as the incident wave and be received by ultrasonic probe 120, it can readily be known that there is a flaw at perpendicular part 116 _(i). Here, the existence of flaws at perpendicular part 116 _(i) can be inspected across the entire circumference of metal pipe bonded body 110 by rotating metal pipe bonded body 110 in one direction.

[0088] Next, the method for measuring the magnitude of a offset that has formed at the inner face end part of the bonded interface shall be described. The offset is measured by the following procedure. First, as shown in FIG. 8, ultrasonic probe 120 is positioned at a prescribed distance from one metal pipe 112 _(i) of the metal pipes that are adjacent each other across the bonded interface of metal pipe bonded body 110. At this time, a contact medium is interposed between metal pipe 112 _(i) and ultrasonic probe 120. Next, an ultrasonic wave is made incident perpendicularly on metal pipe 112 _(i). Since the ultrasonic wave is reflected by the surface and the inner face of metal pipe 112 _(i), the pipe thickness T_(i) of metal pipe 112 _(i) can be measured from the difference in the arrival time of the surface echo and the bottom face echo (first pipe thickness measurement process) Next, ultrasonic probe 120 is moved to the metal pipe 112 _(i+1) side, and the pipe thickness T_(i+1) of metal pipe 112 _(i+1) is measured by the same procedure that was performed for metal pipe 112 _(i) (second pipe thickness measurement process).

[0089] Next, the offset D_(i) that has formed at the surface side of the bonded interface of metal pipe 112 _(i) and metal pipe 112 _(i+1) (this offset shall be referred to hereinafter as the “surface offset”) is measured (surface offset measurement process). The method for measuring the surface offset D_(i) is not restricted in particular. For example, it may be measured using calipers, etc. Also, an ultrasonic wave may be made perpendicularly incident on metal pipe 112 _(i) and metal pipe 112 _(i+1) while keeping the positional relationship between metal pipe bonded body 110 and ultrasonic probe 120 fixed and the surface offset D₁ may also be determined from the difference in the arrival time of the surface echo obtained from metal pipe 112 _(i) and the arrival time of the surface echo obtained from metal pipe 112 _(i+1).

[0090] Next, using the determined pipe thickness T_(i) of metal pipe 112 _(i), pipe thickness T_(i+1) of metal pipe 112 _(i+1), and surface offset D_(i), the offset d_(i) that has formed on the inner face side of the bonded interface (this offset shall be referred to hereinafter as the “inner face offset”) is calculated (inner face offset calculation process). For example, when as shown in FIG. 8, the surface of metal pipe 112 _(i+1) is more protruded than the surface of metal pipe 112 _(i), the inner face offset d_(i) can be determined using the following equation.

d _(i=T) _(i+1)+(T _(i+D) _(i))

[0091] Next, the method for judging the crystalline structure in the vicinity of the bonded interface shall be described The judgment of the crystalline structure is made by the following procedure. First, as shown in FIG. 9, ultrasonic probe 120 is positioned at a prescribed distance from the vicinity of the bonded body part of metal pipe bonded body 110. At this time, a contact medium is interposed between metal pipe 112 _(i) and ultrasonic probe 120. Next, an ultrasonic wave is transmitted in the direction of the interior of metal pipe bonded body 110 at the vicinity of the bonded interface of metal pipe bonded body 110. Here, the “direction of the interior” refers to the direction in which a reflection echo that is reflected from the surface or the bottom face of metal pipe 112 _(i) will not he observed. To be more specific, an ultrasonic wave that is made incident on metal pipe bonded body 110 is made to be transmitted in the circumferential direction, longitudial direction, or a direction intermediate these directions of metal pipe bonded body 110. FIG. 9 shows an example where an ultrasonic wave is made to be transmitted in the circumferential direction of metal pipe bonded body 110. The incidence angle of the ultrasonic wave should be determined in consideration of the sound velocity difference between the contact medium and the metal pipes 112 _(i) and 112 _(i+1).

[0092] Though the incident ultrasonic wave will be transmitted through metal pipe bonded body 110, a portion thereof will be reflected by the grain boundary, reach ultrasonic probe 120 via the same path as the incident wave, and be received by the probe as a backward scattered wave (backward scattering intensity measurement process). Since there is a correlation between the backward scattering intensity and the crystal grain diameter, the crystal grain diameter can be calculated back from the measured backward scattering intensity if this correlation has been determined in advance (crystal grain diameter calculation process).

[0093] Next, the method for inspecting the existence of a solidified phase that has formed at the inner face end part of the bonded interface shall be described. The inspection for the existence of a solidified phase is performed by the following procedure. First, as shown in FIG. 10 ultrasonic probe 120 is positioned at a prescribed distance from metal pipe bonded body 110 and a contact medium is interposed between metal pipe bonded body 110 and ultrasonic probe 120. Next, an ultrasonic wave is made obliquely incident towards the inner face end part of the bonded interface from ultrasonic probe 120.

[0094] If a solidified phase does not exist at the inner face end part of the bonded interface, the incident ultrasonic wave will be reflected by the inner face of metal pipe bonded body 110 as shown by the dotted line in FIG. 10. On the other hand, if there is a solidified phase at the inner face end part of the bonded interface, the incident ultrasonic wave will reach the solidified phase upon transmission through the inner face of metal pipe bonded body 110 and be reflected by the surface of the solidified phase as shown by the solid line in FIG. 10. The position of arrival of the reflected wave will therefore vary depending on the existence of a solidified phase. By measuring this variation of the arrival position by means of another ultrasonic probe 120′, the existence of a solidified phase at the inner face end part of the bonded interface can be judged readily.

[0095] Though embodiments of this invention have been described in detail above, this invention is by no means limited to the above-described embodiments and various modifications are possible within a scope that will not fall outside the gist of this invention. For example, though in all of the above-described embodiments, the various inspections are performed without bringing the ultrasonic probe in close contact with the metal pipe bonded body, the inspections may also be performed upon bringing the ultrasonic probe in close contact with the metal pipe bonded body.

[0096] Also though methods with which various inspections are performed using one ultrasonic probe or one pair of ultrasonic probes were described with the embodiments above, two or more ultrasonic probes or two or more pairs of ultrasonic probes may be used to carry out one type of inspection. Also, two or more ultrasonic probes or two or more pairs of ultrasonic probes may be used to carry out two or more types of inspection at the same time.

[0097] Note that although in the above-mentioned description, the preferred embodiments of a metal-pipe bonded body, a pipe expansion method of metal-pipe bonded body and a method for inspecting metal-pipe bonded body, according to the present invention are separately explained, it is possible for a person skilled in the art to combine these embodiments and utilize the same simultaneous in accordance with needs.

[0098] While there has been described in connection with the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the invention.

[0099] Since the metal-pipe bonded body by this invention is a bonded body made by diffusion bonding a plurality of metal pipes via a bonded interface formed at the end parts of the pipes and at least part of the bonded interface is inclined with respect to the radial direction of the metal pipes, the effect that the formation of cracks at the bonded interface is restrained is provided even in the case where pipe expansion of a high pipe expansion rate is performed.

[0100] In the case where the inner diameter at the vicinity of the bonded interface is made greater than the inner diameter at positions away from the bonded interface, since the shearing force that acts on the bonded interface during pipe expansion is alleviated further, the effect that the formation of cracks at the bonded interface is restrained is provided even in the case where pipe expansion of a high pipe expansion rate is performed.

[0101] Also, with the pipe expansion expanding method of metal-pipe bonded body by this invention, since a expandable metal-pipe bonded body, with which at least part of the bonded interface is inclined with respect to the radial direction of the metal pipes, is used and since the pipe expansion tool is moved from one metal pipe, with which the inclined part of the bonded interface is formed to have a protruding shape, towards another metal pipe, with which the inclined part of the bonded interface is formed to have a recessed shape, the effect that the formation of cracks at the bonded interface is restrained is provided even in the case where pipe expansion of a high pipe expansion rate is performed.

[0102] With this invention, since in the case where a metal pipe bonded body, with which a plurality of metal pipes are liquid phase diffusion bonded via a bonded interface with a inclined part, is inspected by the ultrasonic flaw detection method, an ultrasonic wave is made perpendicularly incident on the inclined part and the reflection echo that has been reflected from the inclined part is detected, the effect of improved precision of detection of flaws that have occurred at the inclined part is provided.

[0103] Also in the case where the bonded interface furthermore has a perpendicular part, the effect of enabling inspection of flaws at the perpendicular part at high precision is provided by the making of an ultrasonic wave obliquely incident on the perpendicular part and detecting the reflection echo that has been reflected from the perpendicular part.

[0104] Furthermore, the effect of enabling the inner face offset to be measured at high precision is provided by the making of an ultrasonic wave perpendicularly incident on a pair of metal pipes that are made adjacent each other via the bonded interface, the measurement of the pipe thickness from the difference in the arrival times of the surface echo and the bottom face echo, and the separate measurement of the surface offset.

[0105] Also, the effect of enabling the crystalline structure in the vicinity of the bonded body part is determined by the making of an ultrasonic wave be transmitted in the direction of the interior of the metal pipe in the vicinity of the bonded interface and the measurement of the backward scattering intensity.

[0106] Furthermore, the effect of enabling inspection for the existence of a solidified phase at the inner face end part in a nondestructive manner is provided by the making of an ultrasonic wave obliquely incident towards the inner face end part of the metal pipe bonded body and the measurement of the position of the reflected wave that has been reflected by the inner face of the metal pipe bonded body. 

What is claimed is:
 1. A metal-pipe bonded body, comprising: first and second metal pipes which are diffusion bonded via a bonded interface defined by end parts thereof, wherein at least part of said bonded interface is inclined with respect to a radial direction of said metal-pipe bonded body.
 2. The metal-pipe bonded body as set forth in claim 1, wherein an inside diameter of said end part of each of said first and second metal pipes is larger than an inside diameter of an intermediate part of each of said first and second metal pipes, said intermediate part being disposed apart from said end part in its longitudinal direction.
 3. The metal-pipe bonded body as set forth in claim 1, wherein said metal-pipe bonded body is an expandable metal-pipe bonded body which is formed by expanding said first and second metal pipes which have been diffusion bonded via said bonded interface.
 4. The metal-pipe bonded body as set forth in claim 3, wherein an inside diameter of said end part of each of said first and second metal pipes is larger than an inside diameter of an intermediate part of each of said first and second metal pipes, said intermediate part being disposed apart from said end part in its longitudinal direction.
 5. The metal-pipe bonded body as set forth in claim 3, wherein said expandable metal-pipe bonded body is formed by expanding said first and second metal pipes which have been diffusion bonded via said bonded interface, with a pipe expansion tool having a tip angle satisfying a following relationship: 0<φ≦θ+60° where (φ) indicates an inclination angle (φ) of said part of said bonded interface and (θ) denotes a half of the tip angle (2θ) of said tool.
 6. The metal-pipe bonded body as set forth in claim 2, wherein an inside diameter of said end part of each of said first and second metal pipes is larger than an inside diameter of an intermediate part of each of said first and second metal pipes, before said first and second metal pipes are subjected to the expansion with said tool, and wherein said intermediate part being disposed apart from said end part in its longitudinal direction.
 7. The metal-pipe bonded body as set forth in claim 1, wherein said inclined part of said bonded interface is subject to an inspection with ultrasonic flaw detection, after said first and second have been diffusion bonded via said bonded interface.
 8. The metal-pipe bonded body as set forth in claim 7, wherein said inspection comprising: a first flaw detection process, in which an ultrasonic wave is made incident perpendicularly on said inclined part and the reflection echo that is reflected from said inclined part is detected.
 9. The metal-pipe bonded body as set forth in claim 8, wherein said ultrasonic wave is a focused ultrasonic wave.
 10. The metal-pipe bonded body as set forth in claim 8, wherein said bonded interface has a perpendicular part which is in parallel with the radial direction of said metal-pipe bonded body, and wherein said inspection further comprises: a second flaw detection process, in which the ultrasonic wave is made obliquely incident on said perpendicular part and the reflection echo that is reflected from said perpendicular part is detected.
 11. A pipe expansion method of metal-pipe bonded body, said bonded body including first and second metal pipes which are diffusion bonded via a bonded interface defined by faced end parts thereof, wherein at least part of said bonded interface is inclined with respect to a radial direction of said metal-pipe bonded body, and said metal-pipe bonded body is enlarged by the insertion of a pipe expanding tool into the interior of the metal pipe bonded body, said pipe expansion method comprising: moving said pipe expanding tool from said first pipe towards said second pipe, wherein said first pipe has one of said end parts protruding towards said second pipe, and said second pipe has the other of said end parts which is recessed so as to receive said protruded end parts of said first pipe.
 12. An inspection method for metal pipe bonded body, said bonded body including first and second metal pipes which are diffusion bonded via a bonded interface defined by faced end parts thereof, wherein at least part of said bonded interface is inclined with respect to a radial direction of said metal-pipe bonded body, said inspection method comprising a first flaw detection process, including the steps of, making an ultrasonic wave incident perpendicularly on said inclined part; and detecting the reflection echo that is reflected from said inclined part.
 13. The inspection method as set forth in claim 12, wherein said ultrasonic wave is a focused ultrasonic wave.
 14. The inspection method as set forth in claim 12, wherein in said first flaw detection process, an ultrasonic probe, which transmits said ultrasonic wave, is scanned in at least one of the circumferential direction and longitudinal direction of said inclined part.
 15. The inspection method as set forth in claim 13, wherein in said first flaw detection process, an ultrasonic probe, which transmits said ultrasonic wave, is scanned in at least one of the circumferential direction and the longituidnal direction of said inclined part.
 16. The inspection method as set forth in claim 12, wherein said bonded interface has a perpendicular part which is extended in parallel with a radial direction of said bonded body, and said method further comprises: as a second flaw detection process including; making an ultrasonic wave obliquely incident on said perpendicular part, and detecting the reflection echo that is reflected from said perpendicular part.
 17. The inspection method as set forth in claim 12, further comprising: a first pipe thickness measurement process, in which an ultrasonic wave is made perpendicularly incident on said first metal pipe and a pipe thickness of said first metal pipe is measured from the difference in the arrival times of a surface echo and a bottom face echo; a second pipe thickness measurement process, in which an ultrasonic wave is made perpendicularly incident on said second metal pipe and a pipe thickness of said second metal pipe is measured from the difference in the arrival times of a surface echo and a bottom face echo; a surface offset measurement process, in which the surface offset between said first and second metal pipes is measured; and an inner face offset calculation process, in which an inner face offset between said first and second metal pipes is determined from the pipe thickness of said first metal pipe, the pipe thickness of said second metal pipe, and said surface offset.
 18. The inspection method as set forth in claim 12, further comprising: a backward scattering intensity measurement process, in which an ultrasonic wave is transmitted in the direction of the interior of said first metal pipe at the vicinity of said bonded interface and the backward scattering intensity is measured, and a crystal grain diameter calculation process, in which a previously determined correlation between the crystal grain diameter of said metal pipe and the backward scattering intensity is used to determine the size of the crystal grain from the magnitude of said backward scattering intensity that was measured in said backward scattering intensity measurement process.
 19. The inspection method as set forth in claim 12, further comprising: a solidified phase detection process, in which an ultrasonic wave is made obliquely incident towards the inner face of said end part of said metal pipe bonded body and the position of the reflection echo that is reflected from the inner face of said metal pipe bonded body is measured. 